112 POWDERED VEGETABLE DRUGS 



DESCRIPTION OF PLATE VI 



FIG. 28. Illustrating the Histology of a Typical Bark Showing all of the Tissues 

 Which May be Found in a Bark. A, Longitudinal section in the radial direction but 

 not showing the medullary rays. B, Transverse section. 1. Outer bark. The de- 

 markation between outer and inner bark is not always distinct. . Inner bark. 3. 

 Beginning of wood tissue, a, Epidermis. Always wanting in tree trunks and older 

 branches, b, Cork tissue, c, Bark parenchyma. Cell-walls are usually not suberized 

 and the cells may contain various inclusions such as crystals of calcium oxalate, tannin, 

 starch granules and resin, d, Groups of sclerenchyma cells. These, when present, 

 normally predominate in the outer bark, e, Crystal-bearing fibers which usually ac- 

 company the bast fibers. /, Bast fibers. These, when present, normally predominate 

 in the inner bark. The fibers may occur singly or in groups, g, Cambium, h, Wood 

 fibers, i, Ducts. Usually of the typically porous type, k, Medullary rays. 



An excellent typical bark having all of the histological elements indicated in Plate 

 VI is Rhamnus purshiana. The demarkation between outer bark and inner bark is 

 well defined in Ulmus and Quillaja. 



